1. Field of the Invention
The present invention relates to a printing blanket in a seamless cylindrical shape which is particularly suitable for use in high-speed printing presses such as high-speed web offset printing presses.
2. Description of the Prior Art
Examples of a printing blanket particularly suitable for high-speed printing presses such as high-speed web offset printing presses include a printing blanket in a seamless cylindrical shape in the circumferential direction thereof.
The above-mentioned printing blanket is constructed by suitably laminating inside a seamless surface printing layer composed of an elastomer such as rubber, a porous and seamless compressible layer composed of an elastomer such as rubber, a non-stretchable layer formed by winding a non-stretchable thread in helical fashion in the circumferential direction, and the like.
A so-called air-type printing blanket having a compressible layer as described above is lower in compressive stress in a nip deformed portion produced by being pressed against a plate cylinder or the like, and is superior in impact absorbability because a variation in the compressive stress caused by the change in the amount of distortion is smaller, as compared with a solid-type printing blanket having no compressible layer, so that it is superior in the effect of preventing impact produced at the time of feeding gears of the printing press, for example, from affecting printing precision.
The solid-type printing blanket causes so-called bulge by stress concentrations on the surface printing layer in the above-mentioned nip deformed portion, which might result in inferior printing such as out of register, inferior paper feeding, double, or deformation of a dot pattern (particularly, dot gain) due to expansion in the circumferential direction. On the other hand, the air-type printing blanket also has the effect of preventing the above-mentioned inferior printing because the compressible layer has the function of lowering stress concentrations on the surface printing layer.
Examples of the above-mentioned compressible layer include a compressible layer having a closed cell structure in which cells are independent of each other, which is formed by expanding matrix rubber using an expanding agent which is decomposed by heating to emit a gas, or blending a hollow microsphere with matrix rubber, for example, and a compressible layer having an open cell structure in which cells connect with each other, which is formed by a so-called leaching method for dispersing in matrix rubber particles, such as common salt particles, extractable by a solvent (water in the case of the common salt particles) which does not affect rubber, vulcanizing the matrix rubber, and then extracting the particles.
However, the compressible layers having the above-mentioned structures respectively have both advantages and disadvantages. The printing blanket having the compressible layer whose structure has the advantage particularly required for printing is forced to be employed by closing our eyes to the disadvantage of the compressible layer.
Specifically, the compressible layer having a closed cell structure (hereinafter referred to as "closed cell compressible layer") is high in tensile strength corresponding to a shear force produced by being pressed against a plate cylinder or the like and therefore, is superior in the property of quickly returning to the original shape after deformation due to compression, that is, stability, as compared with the compressible layer having an open cell structure (hereinafter refereed to as "open cell compressible layer").
Therefore, the printing blanket having the open cell compressible layer exhibits the tendency of the printing pressure to fall below a predetermined value because once the open cell compressible layer has a nip deformed portion produced by being pressed against a plate cylinder, it is not quickly restored by the time the printing blanket makes one revolution to reach the nip deformed portion, thereby causing a so-called permanent set in fatigue; while the printing blanket having the closed cell compressible layer can maintain predetermined printing pressure particularly in high-speed printing because it does not cause the above-mentioned permanent set in fatigue.
The above-mentioned printing blanket having the closed cell compressible layer is quickly restored after being partially, greatly deformed because foreign matter is interposed between the printing blanket and the plate cylinder, thereby making it possible to keep such inferior printing that there are traces of the deformation of the printing blanket due to the foreign matter on prints to a minimum.
However, the closed cell compressible layer is higher in compressive stress in the nip deformed portion and is subjected to a larger variation in the compressive stress caused by the change in the amount of distortion, as compared with the open cell compressible layer, so that it is low in the above-mentioned impact absorbability. The printing blanket having the closed cell compressible layer easily exerts a bad influence on printing precision, for example, by impact produced at the time of feeding gears of the printing press, though the influence is not so large as that in the printing blanket of a solid type.
On the other hand, the open cell compressible layer is low in compressive stress in the nip deformed portion and is superior in impact absorbability because a smaller variation in the compressive stress caused by the change in the amount of distortion is smaller, as compared with the closed cell compressible layer as described above. Accordingly, the printing blanket having the open cell compressible layer is superior in the effect of preventing impact from affecting printing precision.
However, the open cell compressible layer is low in stability as described above. Therefore, the printing blanket having the open cell compressible layer easily causes the above-mentioned permanent set in fatigue and easily causes a large amount of such inferior printing that there are traces of the deformation of the printing blanket by foreign matter on prints.
Furthermore, the open cell compressible layer is low in tensile strength corresponding to a shear force produced by being pressed against a plate cylinder or the like and therefore, easily expands in the circumferential direction. Accordingly, the printing blanket having the open cell compressible layer causes bulge, which might also result in inferior printing such as dot gain due to the expansion in the circumferential direction, though the degree is not so high as that in the printing blanket of a solid type.